The long term objectives of this study are two-fold, namely: 1) to mimic the activation of dihydrogen and dioxygen that occurs at some metalloenzymes in order to better understand these redox transformations in biological systems and, 2) to study energy transfer self-exchange processes which can be used to model fundamental aspects of electron transfer. Studies of redox transformations at metallozymes and of factors affecting electron transfer are of key importance in understanding the physiological electron transfer processes that sustain living organisms. The research design involves utilizing a set of constrained macrocyclic ligands that can be functionalized to provide five-coordinate metal binding environments. The redox chemistry and ligand binding ability of these five-coordinate metal complexes will be analyzed in order to find systems that bind and activate H2 and O2 and may serve as functional mimics of the hydrogenase enzymes and the cytochrome P450 enzymes and the cytochrome P-450 enzymes, respectively. In addition, chromium(III) complexes of the unfunctionalized constrained macrocycles will be prepared and their photophysics closely scrutinized. We plan to use these complexes to continue our studies on energy transfer self-exchange processes, with the aim of determining the role that nuclear and electronic factors play in the exchange rates. A significant amount of literature exists on the similarity of the formalisms between electron transfer and how energy transfer processes can be utilized to model fundamental bimolecular reactions such as electron transfer. However, no systematic studies of energy transfer self-exchange exist due to experimental difficulties. This study will begin to fill that gap. Thus, complexes of a common set of constrained macrocyclic ligands that our laboratory has been working with will serve to further our understanding of electron transfer and redox transformation of small substrates.